Method for determining filament condition by continuously evaluating current flow therethrough



Sept. 9, 1969 METHOD FdR DETERMINING Filed Aug. 19, 1966 I I I LFILAMENT CURRENT/ LIFE I}; 90 '0 -3 g 2700| I 2700K g 80 END OF LIFE-- 32745K a v N 70 2 E 2940K\ o 60 z TIME (HOURS),

IOO I I EFFECT OF VACUUM ON LIFE \ONSTANT VOLTAGE Q5 2700K 2 ii -5 IO mmHg. 3 O

O LU N 3 2 g 0 2 k TIME (HOURS) I 26 CONSTANT VOLTAGE I SUPPLY Q 1 I I6INVENTOR. I E l KURT w WALLACE VOLTAG 32 34 /7 1/1 11/ SUPPLY gm; 36 BYATTORNEY United States Patent 3 466,534 METHOD FOR DETERlVIININGFILAMENT CONDI- TION BY CONTINUOUSLY EVALUATING CUR- RENT FLOWTHERETHROUGH Kurt F. Wallace, Redwood City, Calif., assignor to AmpexCorporation, Redwood City, Calif a corporation of California Filed Aug.19, 1966, Ser. No. 573,670 Int. Cl. G01r 31/22 U.S. Cl. 324-20 1 ClaimABSTRACT OF THE DISCLOSURE The present invention relates to a method andapparatus for determining the end of life of a tungsten hairpin cathodesuch as used in demountable electron optical systems.

The conventional method of ascertaining the end of life of a tungstenhairpin cathode is to either wait for the filament to burn out or tohave a time clock running and make an estimate of the end of life bygoing by previous experience. The main disadvantage of the time clockmethod of determining the filament end of life is that small variationsin temperature of the filament and variations in pressures of the vacuumenclosure have a predominant eifect on the overall life of the filament,and thus the estimated life of a filament due to the particularoperating conditions, may not correspond to the usual known life. Forexample, an error in temperature of 100 C. will cause a decrease in thelife of a filament amounting to 40-50 hours at a particular operatingtemperature.

The present invention overcomes the above shortcomings of the prior artby providing a method and apparatus for increasing the life span of afilament, and for determining the true condition of the filamentirrespective of its operating temperature and pressure.

Accordingly, it is an object of the present invention to provide amethod and apparatus for increasing the life span of a cathode filamentgenerally on the order of times over that of a filament operated underconstant current conditions.

It is a further object of the present invention to provide a means forascertaining the end of life of a tungsten hairpin cathode filament suchas the type used in demountable electron optical systems.

It is another object of the present invention to provide a method andapparatus for determining the true condition of the filament during itsoperating life.

It is yet another object of the present invention to provide anapparatus for operating a filament under conditions of constant voltagewhile continuously monitoring the current flow therethrough to thussense the condition of the filament.

It is still another object of the present invention to provide a methodand apparatus for determining the true condition of a filament bywatching the drop in current due to evaporation of the filament whilemaintaining a constant voltage thereacross.

These and other objects and advantages of the inven- Patented Sept. 9,1969 ice tion will be apparent from the following specification, takenin conjunction with the drawings in which:

FIGURES 1 and 2 are graphs of results obtained in experimental tests,illustrating the normalized current change in filament in relation tooperating life under various conditions of temperature and vacuum,respectively.

FIGURE 3 is a schematic diagram partially in block form showing anembodiment of the apparatus of the present invention.

It is well known in the art that the life of a filament is extended byoperating the filament at constant temperature. In order to obtain suchconditions of constant temperature, the temperature of the filament canbe continuously monitored and fluctuations therein can be corrected byoptimizing the electrical inputs thereto. However, such a process isimpractical for various reasons; i.e., generally because the workingspace in which optical measurements of temperature can be made isinadequate. Therefore prior art systems generally operate the filamentat a constant current in an attempt to approach constant temperatureworking conditions for the filament tip. However, when the filament isrun at constant current, the temperature increases as the filament thinsdue to evaporation and the life-time of the filament is decreased due tothe increased operating temperature.

Rather than attempting to maintain the temperature of the filamentconstant by maintaining the current constant, the present inventionemploys the unique concept of operating the cathode filament at aconstant voltage, which provides a condition very nearly approximatingthe constant temperature conditions desired for filament operation. Byoperating the filament in accordance with the invention concepts it isfound that cathode filament life has been increased by 10 times over thelife of filaments operated under constant current conditions. Inconjunction with operating the filament at constant voltage, thefilament life may be further optimized by preselecting the length of thefilament and the construction surrounding the filament to thus controlthe heat losses therefrom due to radiation and conduction.

Furthermore, as shown in FIGURE 1 by operating the filament at constantvoltage and then monitoring the current, the life of the filament may bepredicted. Thus, it has been found that when the normalized currentshown in FIGURE 2 decreases to the order of the filament is nearing theend of its useful life. The decrease in normalized current shown inFIGURE 1 in essence corresponds to the decrease in the thickness of thefilament as it evaporates, and thus the 20% decrease in normalizedcurrent corresponds essentially to the 10% loss of thickness of thefilament. Accordingly, as taught iby the invention, the evaporation ofthe filament causes a drop in the filament current which in turn is 'avery good indication of the life of the filament. When approaching thearbitrary point, herein chosen by way of example only as 80% of theinitial normalized current, the filament has reached approximately ofits operating life irrespective of the temperature and pressureconditions under which it is operating.

Referring to FIGURE 2 it may be seen that the drop in filament currentindicative of current life is also obtained When the filament is rununder poor vacuum conditions. This may be seen by referring to the curvedepicting the life test of a filament operated at the poor vacuum of 10'mm. of mercury. This curve shows, as does the other curves, that uponapproaching 80% of the normalized current the filament is approachingthe end of life, although it has run for only approximately 10 hours.Thus, even though the poor vacuum conditions may be unsuspected by theoperator, the true condition of the filament, which would generallyoperate much longer than hours, is positively indicated. Accordingly,the present invention provides a filament life modulating scheme whichis much preferred over the usual time clock scheme which does notindicate the vacuum history or temperature history of the filament.

Accordingly, referring now to FIGURE 3 there is depicted apparatuscapable of providing a constant voltage across a cathode filament whileconstantly monitoring the current passed therethrough, to continuouslydetermine the operating condition of the filament and thus indicate atany time the remaining life thereof. By way of example only, there isshown a portion of a conventional electron microscope gun 12 whichincludes an anode 14, a grid 16 and a cathode filament 18. The filament18 is serially connected across a constant voltage supply 20 and afeedback path 22 is provided, whereby the voltage supply to the filamentis regulated. A current meter 24 is connected in series with thefilament 18 and is supplied with a shunting variable resistor 26 havinga manually adjustable slider contact 38. At such time as a new cathodefilament 18 is installed within the electron gun 12, the current meter24 is adjusted to place a dial 30 thereof on the set position, :byadjusting the manually operable slider 28 of the shunt resistor 26. Theelectron gun 12 is maintained at the desired anode-cathode operatingpotentials by means of conventional voltage supply means 32, which isconnected to one input lead to the cathode filament 18. A grid biasmeans 34 is connected from the same side of the cathode filament l8 andthence to the grid 16 within the electron gun 12, to provide the desiredoperating bias for the grid. In the system as shown, the anode 14 isconnected to ground 36 and the voltage supply means 32 comprisesaccordingly a negative source of voltage.

By way of example only, the constant voltage supply 20 may be anyvoltage supply of the conventional type having an output of 23 amps andl.43 volts, which will thus supply a voltage to the filament 18 of aconstant value within the range of from 1.4-1.7 volts. The current meter24 is a conventional current meter reading up to, for example, 3 amps.

Although the present invention has been described herein with regards toa single embodiment it is to be understood that various modificationsmay be made thereto within the scope of the invention.

For example, the constant voltage supply 20 could be .4 replaced by avoltage supply which is not held constant, and a voltmeter could beplaced across the latter supply, any fluctuations in voltage could thenbe compensated for by adjusting the value of a resistor or rheostatplaced in series with the supply. Additionally, the current meter 24could be set upon installation of a new filament 18 by an adjustablescale thereon, rather than by use of a fixed End of Life-Set scale andthe adjustable shunting resistor 26 shown in FIGURE 3. Thus it is notdesired to limit the invention except as defined by the following claim.

What is claimed is: 1. A method for continuously determining thecondition of a replaceable tungsten hairpin cathode filament of the typeused in demountable electron guns during the operation thereofcomprising the steps of:

adjusting the initial current flow through the filament upon assemblythereof within the electron gun to provide an initial reference currentvalue;

determining a selected value of monitored current with respect to theinitial reference current value, such selected value being of the orderof and indicative of the general useful lifespan of the filament;

continuously maintaining a constant operating voltage commensurate withthe initial reference current value across the filament during theoperation thereof; and

monitoring and indicating the current flow generated in the filament inresponse to the applied constant voltage until the indicated currentvalue equals the selected value of 80% of the monitored current, todetermine the degree of evaporation and thus the end-of-life of thefilament and thereby indicate the need for replacement of the filament.

References Cited UNITED STATES PATENTS 2,082,036 6/1937 Stogoff 315-94XR 3,005,949 10/1961 Oakes 324-20 3,040,244 6/1962 Owens 32424 RUDOLPHV. ROLINEC, Primary Examiner p E. L. STOLARUM, Assistant Examiner US.Cl. X.R. 324-62

